The present invention relates to a method for measuring the concentration of a given component in a gas inhaled and/or exhaled by a patient, and to apparatus with which the method can be carried out.
The possibility of determining the composition of the breathing gas in the pulmonary alveoli of a patient is of great interest to the medical field. In intensive care and under anaesthetic, the prime reason for this is because patients could then be more readily supervised and their treatment more favorably adapted. In the field of physiological research, the composition of the breathing gas is determined in order, among other things, to provide improved diagnostic methods. Those components whose concentration in the gas is of primary interest are CO.sub.2, O.sub.2 and gaseous anaesthetics such as N.sub.2 O, halothane, etc. It must be possible to measure the concentration of these components continuously, and preferably with a short response time so as also to enable rapid variations in concentration of the gaseous components of interest to be determined effectively. For example, the amount of oxygen (O.sub.2) consumed by a patient can be determined by measuring the O.sub.2 -content of the breathing gas taken in and expelled by the patient during the inspiration and expiration periods.
The only location where it is practically possible to determine the concentration of a given component of the gaseous mixture inhaled and exhaled by the patient is immediately in front of the mouth of the patient, in the gas line which is connected to the patient's air passages and through which both the inhaled and exhaled gas passes. It should be possible to carry out such a determination with the aid of a transducer disposed in the path of the gas flowing through the aforesaid gas line, at least in the case of certain components of the gas. However, this would place high demands on the measuring transducer, which must be very small and light in weight, besides being capable of withstanding all manner of handling treatment, including cleaning and sterilization. For these reasons, it is preferred to withdraw part of the breathing gas flowing through the line connected to the air passages of the patient, at a location immediately in front of the patient's mouth, and to pass this part flow to a suitable instrument for determining the concentration of the gas component or components of interest in the gaseous mixture. When using this measuring method, however, it is necessary that only a relatively small gas flow is withdrawn and supplied to the measuring instrument. Moreover, the total volume of withdrawn gas between the tapping location and to the measuring instrument must be small, and the time used by the withdrawn gas to reach the measuring instrument must also be short, in order to obtain a rapid measuring response with no risk of different parts of the withdrawn breathing gas mixing together before the gas flow reaches the measuring instrument. Otherwise, it will not be possible to measure rapid variations in the concentration of the gas component of interest in a satisfactory manner.
One difficult problem encountered when carrying out this procedure, is that the relative humidity of the gaseous mixture withdrawn for measurement can vary from nearly 0% to about 97%, while the temperature of the mixture may vary from room temperature to about 35.degree. C. This means that the amount of water carried by the withdrawn gaseous mixture can vary greatly, which leads to all manner of difficulties.
With the total pressure of the gaseous mixture constant, variations in the relative humidity of the gaseous mixture withdrawn for measurement will naturally lead to corresponding variations in the partial pressures of all the other gas components of the mixture. On the other hand, the temperature and the relative humidity in the patient's lungs are both constant. Consequently, the measuring values obtained with respect to the gas component or components of interest must be converted to the conditions prevailing in the lungs of the patient. This requires complicated measurements to be made of the momentary humidity and temperature of the gas mixture supplied to the measuring instrument. It will be understood that this problem exists even though the measuring instrument used is, in itself, insensitive to water vapor, since the variations in the content of water vapor contained in the gas mixture give rise to variations in the contents of all other gas components in the gas mixture, when said mixture is at constant pressure.
The difficulties will, of course, be still greater when the measuring instrument used is sensitive to water vapor, so that measurement of the gas component or gas components of interest is disturbed by the presence of water vapor in the gas mixture. This is the case, for example, with gas concentration detectors incorporating a crystal oscillator whose crystal has a coating which absorbs the gas component or components to be measured, for example a gaseous anaesthetic, and which is also able to absorb water vapor, such that the water-vapor content of the gas mixture will influence the measuring result. Another example is those instruments based on IR-absorption and used for determining, inter alia, CO.sub.2 -contents. These instruments at present use a wavelength of about 4.3 .mu.m, which means that the measuring process will be disturbed by the presence of N.sub.2 O and O.sub.2 in the gas mixture. For this reason, it would be to better advantage if there could be used a wavelength of about 2.6 .mu.m, for which wavelength many good IR-radiation detectors are available. At this wavelength, however, the measuring process is greatly disturbed by variations in the amount of water vapor contained in the gas mixture.
Another difficulty which can occur when the relative humidity and the temperature of the withdrawn gas mixture to be measured are high, is that the water vapor contained in the gas mixture condenses in the pipe or similar line leading to the measuring instrument, and/or in the measuring instrument itself, resulting in clogging of the pipe and damages to the instrument, respectively.
These difficulties could be minimized by drying the gas mixture withdrawn for measurement prior to supplying it to the measuring instrument, either by causing the water vapor in the gas mixture to condense and collecting the condensation in a water trap, or by passing the gas mixture through a suitable drying agent capable of absorbing the water vapor in the gas mixture, so that in both cases a substantially dry gas mixture is supplied to the measuring instrument. Both of these solutions to the problem, however, have been found in practice to be either unusable or highly unsuitable. For example, the water trap or the drying device must be regularly superintended, a task which is considered troublesome by those using the measuring equipment. A more serious disadvantage with these solutions, however, is that the presence of a water trap or a drying device results in an increase in the volume of gas between the tapping location and the measuring instrument and also in the time taken for the gas mixture to pass from the tapping location to said measuring instrument, which, as mentioned in the aforegoing, results in a lengthening of the measurement response time, so that rapid variations in the concentration of the gas component of interest cannot readily be detected. This problem can only be counteracted by increasing the flow of gas withdrawn for measurement. On the other hand, such an increase in the withdrawn gas flow is not desirable, since only a small part of the total volume of gas inhaled and/or exhaled by the patient should be withdrawn for measuring purposes; and said total volume may, in itself, be small, such as when treating children for example.